How do I create a two sided plane with subsurface scattering for modelling leaves and foliage?
You don't :).
Leaves have no Subsurface Scattering (which needs greater thickness), they only have Translucency (also called Diffuse Scattering).
The linked devtalk post already offers a setup for correct backface reflections.
To use different texture for Frontface/Backface, split the node-tree into two branches.




Result with a small light underneath to show translucency
 


For convenience, I packed the whole setup into a nodegroup called Principled Leaves BSDF.
You can download it below. Both front/back images are packed in the file.